paleomagnetism

Polar wandering curves for the various continents
through geological time based on paleomagnetic data. The numbers refer
to: 1. Precambrian; 2. Cambrian; 3. Ordovician; 4. Silurian; 5. Devonian;
6. Carboniferous; 7. Permian; 8. Triassic; 9. Jurassic; 10. Cretaceous;
11. Tertiary; 12. Quaternary. Since these curves, which describe the
apparent positions of the north magnetic pole as determined from different
continents, are not only colinear but also show remarkable differences
of form, the only apparent explanation of polar wandering would seem
to be that the continents have moved relative to each other

The study of past changes in the Earth's magnetic field (see geomagnetic
field) by examination of rocks containing certain iron-bearing minerals
(e.g., hematite, magnetite).
Since the "magnetic memory" of rocks is measurable, this determines their
orientation in relation to magnetic north at the time of their solidification.
The Earth's polarity has reversed at least 20 times in the past 4 to 5 million
years; earlier changes cannot at present be determined. Reversals of the
field and movements of the magnetic poles can be charted and information
on continental drift may be obtained.
The gross displacement of large rock formations as measured by their magnetic
qualities can be explained by seafloor
spreading.